Hearing impairment can be characterized according to its physiological source. There are two general categories of hearing impairment, conductive and sensorineural, although sometimes a combination of both may arise (mixed). Conductive hearing impairment results from diseases or disorders that limit the transmission of sound through the outer and/or middle ear. Conductive hearing impairment is often treated surgically with an implanted prosthesis to replace part or all of the ossicles. Sensorineural hearing impairment occurs in the inner ear and/or neural pathways and accounts for the vast majority of hearing impairment. In patients with sensorineural hearing impairment, the external and middle ear function normally. The sound vibrations are transmitted undisturbed through the eardrum and ossicles where fluid waves are created in the cochlea. However, because some of the delicate sensory hair cells inside the cochlea have degenerated or been damaged, the inner ear cannot detect the full intensity and quality of the sound. Acquired sensorineural hearing loss can occur as a result of aging or exposure to loud noise over a protracted period of time.
Hearing loss is quantified as borderline or slight, mild, moderate, severe, and profound, according to the volume of sounds that can be heard without amplification. Individuals with severe hearing loss are unable to hear sound below 70 dB and individuals with profound hearing loss are unable to hear sound below 91 dB. In practical terms, a person with severe hearing loss is incapable of hearing sounds softer than that of a busy street, while a person with profound hearing loss is incapable of hearing sounds softer than that of a subway. Without any type of assistance, normal conversation is not heard by individuals in either group. Over half a million Americans are suffering from severe to profound hearing loss (Mohr et al., 2000, Int. J. Technol. Assess Health Care, 16:1120-1135).
Tinnitus or ringing in the ears is another frequently debilitating auditory problem characterized by the perception of ringing, hissing, or other sound in the ears or head when no external sound is present. About 12 million Americans have severe enough tinnitus to seek medical attention, and about two million of these are seriously incapacitated by this condition. Untreated hearing impairments result in lost productivity, and increased medical and educational expenditures. Thus, apart from consequences to the individual, hearing loss is costly to society.
Traditional approaches to the management of sensorineural hearing impairment and tinnitus involve the use of hearing aids or cochlear implants. Hearing aids are commonly used to manage the mild to severe sensorineural hearing-impaired population. Cochlear implants are indicated for individuals with severe to profound hearing loss. Hearing aids are devices that acoustically amplify sound to increase the movement of the tympanic membrane and indirectly vibrate the ossicles. Cochlear implants electronically stimulate the inner ear through a series of electrodes implanted in the cochlea during a surgical procedure.
However, stimulation of structures of the ear only acoustically or electronically, is not sufficient to provide optimal hearing correction and is frequently accompanied by undesirable side effects. For instance, traditional hearing aids provide limited hearing assistance in the high frequency range, whereas, cochlear implants provide limited hearing assistance in the low frequency range. In addition, traditional hearing aids do not always produce clear natural sounds, and cochlear implants do not always correct and may even aggravate tinnitus.
Thus, there remains a need in the art for medical devices that provide hearing correction across the entire normal audio frequency range for individuals with severe to profound hearing loss or tinnitus. The present invention provides partially and fully implantable hearing devices that provide suitable stimulation to structures of the ear resulting in superior hearing correction.